1. Field of Invention
The present invention relates to a method of forming an optical device. More particularly, the present invention relates to a method of forming a color filter array (CFA).
2. Description of Related Art
Color filter array (CFA) is frequently used in optical devices for displaying color images. In such an optical device, each pixel generally corresponds to three neighboring but different color filters. In other words, a specified color is permitted to pass through each filter. A complete color palette can be obtained, for example, by mixing the colors from the three filters such as red (R), green (G) and blue (B). Therefore, the pixel array in an image display device actually consists of an array of three types of color filters such as an array of red filters, an array of green filters and an array of blue filters.
The fabrication of an optical device that displays colorful images normally involves forming a passivation layer such as a silicon oxynitride (SiON) layer over an array of background light sensitive devices. Thereafter, the three color filter arrays are independently fabricated over the passivation layer.
FIGS. 1A through 1C are schematic cross-sectional views showing the progression of steps for fabricating a conventional tricolor filter array. First, as shown in FIG. 1A, a substrate 100 having an array of metallic layers 110 thereon, in which the metallic layers 100 serve as light-sensitive elements, is provided. A passivation layer 120 such as a silicon oxynitride layer is formed over the substrate 100 and the metallic layers 110. A red negative photoresist layer 130 is formed over the passivation layer 120. An i-line light source (365 nm) 132 shines on the photomask 134 and exposes a portion of the red photoresist layer 130. Ultimately, cross-linking of high molecular weight polymer occurs in a light-exposed region 136 of the red photoresist layer.
As shown in FIG. 1B, a chemical development of the red photoresist layer is carried out so that red photoresist material outside the exposed region 136 is removed. Thus, a red color filter 140 is produced.
As shown in FIG. 1C, the steps illustrated in FIG. 1B are repeated twice to form a green color filter 150 and a blue color filter 160, respectively. Note that only one red/green/blue color filter (140/150/160) is shown in the figure. In fact, an array of red/green/blue color filters covering a wide area is formed.
However, the conventional method of fabricating a color filter array has several problems. First, an I-line light source 132 having a wavelength of 365 nm is used to carry out exposure. However, the silicon oxynitride layer is a poor absorber for light of this wavelength and hence cannot absorb reflected light 132a coming from a lower layer. Consequently, a portion of red photoresist 130 outside the desired exposure region 136 is also exposed leading to the formation of cross-linked polymers. After a post-exposure development, a residual patch 130a of red photoresist remains over the passivation layer 120. In the presence of a residual patch 130a, subsequent fabrication of the green filter 150 and the blue filter 160 are also affected. Ultimately, image quality of the device will deteriorate.
Second, the conventional method of forming a blue color filter array often leads to color pixel peeling due to insufficient light exposure. FIGS. 2A and 2B are schematic cross-sectional views showing a conventional technique for forming a blue color filter array on a substrate and resulting adverse consequence. As shown in FIG. 2A, an i-line light source 232 shines through a photomask 234 and exposes a portion of the blue negative photoresist layer 230 on the passivation layer 220. Since blue negative photoresist has a relatively high i-line absorption coefficient, lower edges of the exposed region 236 in the photoresist layer 230 may not receive sufficient i-line light to trigger cross-linking. Consequently, as shown in FIG. 2B, the blue color filter 240 is narrower at its base than at its top after photoresist development. Hence, the chance of this blue color filter 240 peeling off increases considerably (the dashed lines indicate the position of the blue filter before peeling).
In addition, resolution of a light source with a wavelength of 365 nm is relatively low and hence area utilization is unsatisfactory. Furthermore, resolution of the color filters is ultimately limited by the resolution of the light source.
Accordingly, one object of the present invention is to provide a method of forming a color filter array. A substrate having a passivation layer such as a silicon oxynitride layer thereon is provided. A negative color photoresist layer is formed over the passivation layer. A photolithographic exposure process is conducted using a light source with a wavelength less than or equal to 248 nm so that a pattern for forming the color filter array is imprinted on the negative color photoresist layer.
In the aforementioned photolithographic exposure, a light source having a wavelength less than or equal to 248 nm is used in photolithographic exposure. Since silicon oxynitride material has a higher light absorption capacity for light at such a wavelength, the passivation layer actually serves also as an anti-reflection coating that improves the final quality of the patterned color filter array.
In addition, by using a light source having a wavelength less than or equal to 248 nm rather than an i-line light source, resolution of the photolithographic process is increased. Ultimately, both area utilization and resolution of the color filter panel improves.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.